Method of increasing the low shear rate viscosity of well drilling and servicing fluids containing calcined magnesia bridging solids, the fluids and methods of use

ABSTRACT

The invention provides well drilling and servicing fluids, and methods of drilling, completing, or working over a well therewith, preferred fluids comprise an aqueous liquid, a water soluble polymer viscosifier (preferably xanthan gum), a polymeric fluid loss control additive (preferably a partially depolymerized partially crosslinked hydroxyalkyl ether derivative of starch or a hydroxyalkyl ether derivative of a partially crosslinked and partially depolymerized starch), a bridging agent comprising a particulate calcined magnesia that has an Activity Index greater than about 800 seconds, and citric acid in an amount sufficient to increase the low shear rate viscosity of the fluids. The invention further provides a method of increasing the low shear rate viscosity of fluids which contain a water soluble polymer and a calcined magnesia bridging agent which has an Activity Index greater than about 800 seconds.

This patent application is a continuation-in-part of patent applicationSer. No. 10/411,640 filed Apr. 11, 2003, the entire contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to clay-free aqueous well drilling andservicing fluids, methods of preparation thereof, and methods ofdrilling or servicing a well therewith.

The use of fluids for conducting various operations in the boreholes ofoil and gas wells which contact a hydrocarbon-containing subterraneanformation are well known. Thus, drill-in fluids are utilized wheninitially drilling into potential hydrocarbon producing formations.Completion fluids are utilized when conducting various completionoperations in the hydrocarbon-containing formations. Workover fluids areutilized when conducting workover operations of previously completedwells.

It is important that the fluids which contact hydrocarbon-containingformations are formulated such that there is a minimum penetration offluid, both the aqueous phase and the solid phase, into the formation.Thus, the present state-of-the-art fluids generally comprise a “watersoluble” polymer, preferably a biopolymer such as xanthan gum orscleroglucan gum, starch derivatives for fluid loss control, and watersoluble or acid soluble bridging agents to form a thin filter cake whichforms a protective seal of the formation. See for example the followingU.S. Patents, incorporated herein by reference: Mondshine U.S. Pat. No.4,620,596; Dobson, Jr. et al. U.S. Pat. No. 4,822,500; Dobson, Jr. etal. U.S. Pat. No. 5,629,271; Dobson, Jr. et al. U.S. Pat. No. 5,641,728;Dobson, Jr. et al. U.S. Pat. No. 5,728,652; and Dobson, Jr. et al. U.S.Pat. No. 5,804,535. A recent development is a biopolymer-free fluidwhich utilizes a unique amylopectin starch derivative for both viscositydevelopment and fluid loss control as set forth in Dobson, Jr. et al.U.S. Pat. No. 6,391,830.

After the well has been drilled and completed, it is necessary to removethe filter cake from the surface of the formation allowing thehydrocarbons therein to flow to the wellbore for production. This isgenerally aided by contacting the filter cake with various washes/soaksolutions in which the components of the filter cake are soluble, mostgenerally acidic aqueous fluids. See, for example, the following U.S.patents, incorporated herein by reference: Mondshine et al. U.S. Pat.No. 5,238,065; Dobson, Jr. et al. U.S. Pat. No. 5,607,905; Dobson, Jr.et al. U.S. Pat. No. 5,783,527; and Dobson, Jr. et al. U.S. Pat. No.5,783,526.

As indicated in Mondshine U.S. Pat. No. 4,620,596, sparingly solubleborates have been utilized as bridging agents in well drilling andservicing fluids. However, one problem with their use inbiopolymer-containing fluids is the crosslinking of the biopolymers thatoccurs when the borate anion reacts with the biopolymers. Thus, there isa need for another bridging agent that is sparingly soluble inwater/aqueous systems and is soluble in, acidic solutions.

Powdered magnesium oxide is utilized in the art as an alkalinity controladditive for biopolymer-containing fluids as exemplified by the U.S.patents referenced hereinbefore.

The magnesium oxide as referenced in Dobson, Jr. et al. U.S. Pat. No.5,514,644, incorporated herein by reference, has an Activity Index lessthan about 100 seconds, most preferably less than about 50 seconds.

Disclosed in co-pending patent application Ser. No. 10/411,540 filedApr. 11, 2003 is the use of calcined magnesia as a bridging agent inpolymer-containing well drilling and servicing fluids.

The calcined magnesia provides biopolymer-containing well drilling andservicing fluids which do not gel on thermal aging at temperatures atwhich the biopolymer does not decompose and which utilizes theparticulate, sized magnesia particles as a bridging agent to form therequired thin filter cake to limit fluid invasion into thehydrocarbon-containing formation contacted by the fluid.

The present invention pertains to stable well drilling and servicingfluids which provide a filter cake that is partially water soluble andsubstantially acid soluble for improved removal from the sides of theborehole/face of the hydrocarbon-containing formations on which thefilter cake is deposited and a method of increasing the low shear rateviscosity thereof.

SUMMARY OF THE INVENTION

The present invention provides a stable water soluble polymer-containingwell drilling and servicing fluid which utilizes as a bridging agentparticulate, sized magnesia which has an Activity Index greater thanabout 800 seconds and citric acid to increase the low shear rateviscosity thereof.

The present invention further provides a method of drilling a wellwherein there is circulated within the wellbore being drilled asdrilling proceeds a water base drilling fluid containing as a bridgingagent particulate, sized magnesia which has an Activity Index greaterthan about 800 seconds and citric acid to increase the low shear rateviscosity thereof.

The present invention further provides a process of completing orworking over a well wherein a subterranean formation is contacted withan aqueous fluid wherein the fluid contains a bridging agent comprisinga particulate, sized magnesia which has an Activity Index greater thanabout 800 seconds and citric acid to increase the low shear rateviscosity thereof.

The invention still further provides a method of increasing the lowshear rate viscosity of water soluble polymer-containing well drillingand servicing fluids which comprises adding to the fluids citric acid inan amount sufficient to increase the low shear rate viscosity whereinthe fluids contain as a bridging agent a calcined magnesia which has anActivity Index greater than about 800 seconds.

Other objects, features and embodiments of the invention are disclosedin the following description of the invention and appended claims.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof will hereinafter bedescribed in detail and shown by way of example. It should beunderstood, however, that it is not intended to limit the invention tothe particular forms disclosed, but, on the contrary, the invention isto cover all modifications and alternatives falling within the spiritand scope of the invention as expressed in the appended claims.

The compositions can comprise, consist essentially of, or consist of thestated materials. The method can comprise, consist essentially of, orconsist of the stated steps with the stated materials.

DETAILED DESCRIPTION OF THE INVENTION

It is well known in the oil and gas well drilling and servicing art toemploy aqueous well drilling and servicing fluids (hereinafter sometimesreferred to as “WDSF”) which exhibit an elevated low shear rateviscosity (hereinafter sometimes referred to as “LSRV”). Such fluids arepseudoplastic, shear thinning fluids and are particularly preferredfluids when conducting horizontal or directional drilling or wellservicing operations in boreholes. See for example Dobson, Jr. et al.U.S. Pat. No. 5,804,535.

We have now found that the LSRV of WDSF comprising an aqueous phase, awater soluble polymer viscosifier/suspension agent, and a calcinedmagnesia bridging agent having an activity index greater than about 800seconds can be increased by adding thereto citric acid.

As indicated, the WDSF of this invention comprise an aqueous phase, awater soluble polymer viscosifier/suspension agent, a calcined magnesiabridging agent having an activity index greater than about 800 seconds,and citric acid.

The Activity Index of magnesia is obtained using the following apparatusand test procedures.

The rate at which magnesium oxide reacts with a dilute solution ofacetic acid is used as a measure of activity. An excess of magnesia isused so that at the end point of the reaction, the solution goes fromacidic to basic and is detected by a color change employingphenolphthalein indicator.

Apparatus and Reagents

-   -   Acetic acid solution 1.00±0.01N, standardized    -   Phenolphthalein soln. (1% solution in ethanol)    -   Waring blender, 2 speed with 32 oz. glass container    -   Balance with sensitivity of 0.01 gm    -   Stopwatch    -   Thermometer    -   Graduated cylinders, 100 ml and 500 ml    -   Procedure

1. Prior to the test, the water and the acetic acid solution should bebrought to a temperature of 25±1 C.

2. Weigh a 5.00±0.02 grams aliquot of the magnesia sample.

3. Measure out 300 ml of water in a graduated cylinder and add it to theblender.

4. Carefully hold a thermometer in the blender and run blender until thetemperature of the water is 28 C. Turn off the blender.

5. Add 5-10 drops of phenolphthalein indicator solution.

6. Add the magnesia sample and immediately start the blender on lowspeed.

7. Count ten seconds from the start of the blender and add 100 ml of the1.00N acetic acid solution. The stopwatch is started as the acid isbeing added.

8. Stop the timer when the solution turns to a definite pink color.Record the reaction time in seconds as the activity index of themagnesia

9. Note: Add three to five additional drops of indicator solution to theblender every 30 seconds until the color change has taken place.

Magnesia having an Activity Index less than about 800 seconds is toowater soluble producing biopolymer-containing fluids which becomegelatinous on heating.

The preferred WDSF of the invention comprise one or more polymerviscosifier/suspension agents, one or more polymeric fluid loss controlagents, the calcined magnesia bridging agent, and citric acid dispersedin an aqueous liquid.

The preferred polymer viscosifier is a biopolymer (microbialpolysaccharide). The term “biopolymer” is intended to mean an excellularpolysaccharide of high molecular weight, in excess of about 500,000,produced by fermentation of a carbohydrate source by the action ofbacteria or fungi. Representative microorganisms are the genusXanthomonas, Pseudomonas, Agrobacterium, Arthrobacter, Rhizobium,Alcaligenes, Beijerincka, and Sclerotium. A scleroglucan typepolysaccharide produced by microorganisms such as NCIB 11592 and NCIB11883 is commercially available from Degussa.

The preferred biopolymer viscosifier useful in the practice of thisinvention is preferably a xanthomonas gum (xanthan gum). Xanthomonas gumis available commercially from Rhodia under the tradename VISULTRA. Itis a widely used viscosifier and suspending agent in a variety offluids. Xanthomonas gum can be made by the fermentation of carbohydratewith bacteria of the genus Xanthomonas. Representative of these bacteriaare Xanthomonas campestris, Xanthomonas phaseoli, Xanthomonasmulvacearn, Xanthomonas carotoe, Xanthomonas traslucens, Xanthomonashederae, and Xanthomonas papavericoli. The gum produced by the bacteriaXanthomonas campestris is preferred for the purpose of this invention.The fermentation usually involves inoculating a fermentable brothcontaining a carbohydrate, various minerals and a nitrogen yieldingcompound. A number of modifications in the fermentation procedure andsubsequent processing are commercially used. Due to the variety offermentation techniques and difference in processing operationsubsequent to fermentation, different production lots of xanthomonas gumwill have somewhat different solubility and viscosity properties.Xanthomonas gums useful in the practice of the present invention arerelatively hydratable xanthomonas gums.

Xanthan gum is a polymer containing marmose, glucose, glucuronic acidsalts such as potassium glucuronate, sodium glucuronate, or the like,and acetyl radicals. Other Xanthomonas bacteria have been found whichproduce the hydrophilic gum and any of the xanthan gums and theirderivatives can be used in this invention. Xanthan gum is a highmolecular weight linear polysaccharide that is readily soluble in waterto form a viscous fluid.

Other biopolymers prepared by the action of other bacteria, or fungi, onappropriate fermentation mediums may be used in the fluids of thepresent invention provided that they impart the desired thermally stablerheological characteristics thereto. This can be readily determined byone skilled in the art in accordance with the teachings of thisspecification.

Polymeric fluid loss control additives used in well drilling andservicing fluids are so-called water soluble polymers includingpregelatinized starch, starch derivatives, cellulose derivatives,lignocellulose derivatives, and synthetic polymers. Representativestarch derivatives include: hydroxyalkyl starches such as hydroxyethylstarch, hydroxypropyl starch, hydroxypropyl carboxymethyl starch, theslightly crosslinked derivatives thereof, and the like; carboxymethylstarch and the slightly crosslinked derivatives thereof; cationicstarches such as the tertiary aminoalkyl ether derivatives of starch,the slightly crosslinked derivatives thereof, and the like.Representative cellulose derivatives include low molecular weightcarboxymethyl cellulose, and the like. Representative lignocellulosederivatives include the alkali metal and alkaline earth metal salts oflignosulfonic acid and graft copolymers thereof Representative syntheticpolymers include vinyl sulfonate copolymers, and polymers containingother sulfonate monomers.

The preferred polymeric fluid loss control additives used in theinvention are the starch ether derivatives such as hydroxyethyl starch,hydroxypropyl starch, dihydroxypropyl starch, carboxymethyl starch,hydroxyalkyl carboxymethyl starch, and cationic starches, and theslightly crosslinked derivatives of these starch ethers, 15 mostpreferably the hydroxypropyl ether derivative of starch and the slightlycrosslinked derivatives thereof.

Most preferably the polymeric fluid loss control additive is a starchether derivative which has been slightly crosslinked, such as withepichlorohydrin, phosphorous oxychloride, soluble trimetaphosphates,linear dicarboxylic acid anhydrides, N,N¹-methylenebisacrylamide, andother reagents containing two or more functional groups which are ableto react with at least two hydroxyl groups. The preferred crosslinkingreagent is epichlorohydrin. Generally, the treatment level is from about0.005% to about 0.1% of the starch to give a low degree of crosslinkingof about one crosslink per 200 to 1000 anhydroglucose units. Thecrosslinking may be undertaken before or after the starch isderivatized. Additionally, the starch may be modified by acid or enzymehydrolysis or oxidation, to provide a lower molecular weight, partiallydepolyermized, starch polymer for derivatization. Alternatively, thestarch ether derivative may be modified by acid hydrolysis or oxidationto provide a lower molecular weight starch ether derivative. The bookentitled “Modified Starches: Properties and Uses,” by O. B. Wurzburg,1986 (CRC Press, Inc., Boca Raton, Fla., U.S.A.) is an excellent sourcefor information in the preparation of starch derivatives.

Still most preferably, the polymeric fluid loss additive is a starchderivative selected from the group consisting of (1) a crosslinked etherderivative of a partially hydrolyzed starch, (2) a partiallydepolymerized, crosslinked ether derivative of starch, and (3) mixturesthereof, as set forth in Dobson, Jr. et al. U.S. Pat. No. 5,641,728,incorporated herein by reference, commercially available as BROMA FLA™from TBC-Brinadd, Houston, Tex.

In case (1) the starch is partially depolymerized prior to crosslinkingand derivatizing the starch, whereas in the latter case (2) the starchis first crosslinked and derivatized prior to partially depolymerizingthe starch derivative. In either case, the molecular weight of thecrosslinked starch derivative is decreased by the partialdepolymerization of the starch polymer. As used throughout thisspecification and claims, the terms “partially depolymerized starchderivative,” and “hydrolyzed starch derivative” and the like areintended to mean the starch derivatives prepared by either case (1) orcase (2).

In case (1), it is preferred that the starch be hydrolyzed ordepolymerized to the extent that the viscosity of an aqueous dispersionof the starch is reduced about 25% to about 92%, preferably about 50% toabout 90%, prior to crosslinking and derivatizing the starch. In case(2), it is preferred that the crosslinked starch derivative behydrolyzed or depolymerized to the extent that the viscosity of a waterdispersion of the starch derivative at a concentration of 60 kg/m³ isreduced about 15% to about 50%, preferably about 20% to about 40%.

Patents which disclose oxidative processes for partially depolymerizingstarch derivatives and/or starches include the following, incorporatedherein by reference: U.S. Pat. No. 3,975,206 (Lotzgesell et al.); U.S.Pat. No. 3.935,187 (Speakman); U.S. Pat. No. 3,655,644 (Durand). Patentswhich disclose acidic processes for partially depolymerizing starchderivatives and/or starches include the following, incorporated hereinby reference: U.S. Pat. No. 3,175,928 (Lancaster et al.); U.S. Pat. No.3,073,724 (Rankin et al.). Reference information on the acidmodification of starches is presented in “Starch: Chemistry andTechnology” 2nd Edition, 1984, Roy L. Whistler, James N. Bemiller andEugene F. Paschall, editors, Chapter XVII, pp. 529-541, “Acid-ModifiedStarch: Production and Uses.”

The partially depolymerized or hydrolyzed starch in case (1) or thestarch in case (2) is crosslinked with a compound the molecules of whichare capable of reacting with two or more hydroxyl groups. Representativecrosslinking materials are epichlorohydrin and other epihalohydrins,formaldehyde, phosphorous oxychloride, trimetaphosphate, dialdehydes,vinyl sulfone, diepoxides, diisocyanates, bis(hydroxymethyl) ethyleneurea, and the like. The preferred crosslinking compound isepichlorohydrin. Crosslinking of the starch (or hydrolyzed starch)results in an increase in the molecular weight of the starch and anincrease in the viscosity of aqueous dispersions of the starch.

The reaction conditions used in making crosslinked starches vary widelydepending upon the specific bi-or polyfinctional reagent used for thecrosslinking. In general, most of the reactions are run on aqueoussuspensions of starch at temperatures ranging from room temperature upto about 50° C. Often an alkali such as sodium hydroxide is used topromote reaction. The reactions are normally run under neutral to fairlyalkaline conditions, but below the level which will peptize or swell thestarch. If the crosslinking reaction is run in an aqueous suspension ofstarch, when the desired level of crosslinking (usually as measured bysome type of viscosity or rheology test) is reached, the starchsuspension is neutralized and the starch is filtered and washed toremove salts, any unreacted reagent, and other impurities produced byside reactions of the crosslinking reagent with water. Konigsberg U.S.Pat. No. 2,500,950 discloses the crosslinking of starch withepoxyhalogen compounds such as epichlorohydrin.

It is preferred that the starch or hydrolyzed starch for use in thepresent invention be crosslinked with epichlorohydrin in a basic aqueousstarch suspension at a temperature and for a period of time such thatthe Brabander viscosity of the suspension is within about 50% to 100% ofthe maximum viscosity. The viscosity will vary by the amount ofcrosslinking and the test conditions, i.e., temperature, concentrations,etc. A viscosity peak indicates maximum crosslinking. When the desiredviscosity is reached, the crosslinking reaction is terminated. ABrabender viscometer is a standard viscometer readily available on theopen market and well known to those skilled in the art.

Generally, the treatment level is from about 0.005% to about 0.1% ofstarch to give a low degree of crosslinking of about one crosslink per200 to 1000 anhydroglucose units. As indicated, the crosslinking may beundertaken before or after the starch is derivatized.

The epichlorohydrin crosslinked starch is then preferably reacted withpropylene oxide to form the hydroxypropyl ether. The reaction ofpropylene oxide and starch is base catalyzed. Aqueous slurry reactionsare generally catalyzed by 0.5 to 1% sodium hydroxide based on the dryweight of starch. Sodium sulfate or sodium chloride may be added to keepthe starch from swelling during reaction with the propylene oxide.Reaction temperatures are generally in the range of from about 37.7° C.to about 51.7° C. (100° to 125° F.). Propylene oxide levels generallyrange from about 1% to about 10% based on the dry weight of the starch.Propylene oxide-starch reactions take approximately 24 hours to completeunder the conditions described and are about 60% efficient with respectto the propylene oxide. It is preferred that the epichlorohydrincrosslinked hydroxypropyl ether contain from about 0.5% to about 5%reacted propylene oxide based on the dry weight of starch or hydrolyzedstarch.

Other methods of preparing epichlorohydrin crosslinked starches andhydroxypropyl starch ethers are well known in the art.

The preferred starch ether derivative as indicated is the hydroxypropylether. Other representative starch derivatives are hydroxyethyl ethers,carboxymethyl ethers, dihydroxypropyl ethers, hydroxyalkyl carboxymethylethers, and cationic starch ethers. The preparation of such starchderivatives is well known in the art.

The particle size distribution of the calcined magnesia bridging agentmust be sufficient to bridge across and seal the pores in thesubterranean formation contacted by the fluid. Generally, as disclosedin U.S. Pat. No. 4,175,042, incorporated herein by reference, theparticle size range is from about 5 microns to about 800 microns withgreater than about 5% by weight of the particles being coarser thanabout 44 microns. However, as indicated in Dobson, Jr. et al. U.S. Pat.No. 5,629,271, incorporated herein by reference, the addition of asupplementary bridging agent having a particle size such that at least90% of the particles thereof are less than 10 microns and the averageparticle size is from about 3 to about 5 microns decreases the fluidloss of the fluids and reduces the concentration of polymer required toimpart the desired degree of fluid loss control to the fluids. This ineffect increases the concentration of particles less than 10 micronsdiameter in the fluid.

Since the particle size distribution of the bridging agent needed in anywell drilling and servicing operation is related to the size of theopenings in the formations to be bridged and sealed, it is preferred tohave several particulate, sized magnesia products having differentparticle size distributions which can be blended to produce fluidseffective in sealing the formations contacted by the fluids.

The aqueous liquid used to prepare the WDSF of this invention may be anyliquid compatible with the polymeric viscosifier and the polymeric fluidloss control additive used to prepare the WDSF. Thus, the aqueous liquidmay be natural or a synthetic brine having one or more water solublesalts dissolved therein. Exemplary water soluble salts well known in theart are sodium chloride, calcium chloride, potassium chloride, sodiumbromide, calcium bromide, potassium bromide, zinc bromide, sodiumformate, potassium formate, cesium formate, and other water solublesalts as desired. Generally, the concentration of water soluble salts inthe aqueous brine may be any concentration up to saturation in order toprovide the aqueous liquid with the density desired, such as from 8.3ppg (1000 kg/m³) to about 19.2 ppg (2304 kg/m³).

The fluids of this invention are further characterized in Table A. TABLEA Most Operable Preferred Preferred Water Soluble Polymer Viscosifier,0.5-5   0.75-4   1-3 ppb Fluid Loss Control Additive, ppb  2-15  3-104-8 Calcined Magnesia Bridging Agent,  15-100 20-80 25-60 ppb CitricAcid, ppb 0.05-5   0.1-4   0.15-3   Low Shear Rate Viscosity,cp* >10,000 >15,000 >20,000 Spurt Loss, ml*, ** <5 <3 <3 30-Minute FluidLoss, ml*, ** <15 <10 <10*Determined as disclosed hereinafter**The preferred fluids containing a polymeric fluid loss control agent

The fluids of the invention may be prepared and the method of theinvention practiced, by mixing the aqueous liquid as set forth hereinwith the polymeric viscosifier, the polymer fluid loss control additiveif present, the bridging agent, the citric acid, and any optionaladditives as desired.

The fluids of the invention are useful in various petroleum recoveryoperations such as well drilling, including drilling intohydrocarbon-containing formations, completion, workover and the like allas are well known in the art. Specifically the fluids of the inventionare useful in drilling a well wherein the drilling fluid is circulatedwithin a borehole being drilled as drilling proceeds, and in wellcompletion and workover methods wherein a subterranean formation iscontacted with an aqueous fluid to form a bridge and seal on theformation, all as are well known in the art.

The low shear rate viscosity (LSRV) for purposes of this invention isobtained using a Brookfield Model LVTDV-1 viscometer having a number 1or 2 spindle at 0.3 revolutions per minute (shear rate of 0.0636 sec⁻¹).The fluid loss characteristics of the fluids are obtained by a modifiedAPI filtration test. Thus, to an API high temperature filtration cellwith removable end cages is added a 10 micron disk (i.e., an aluminumoxide Aloxite™ ceramic disk having 10 micron pore throats, from 600 to750 md permeability, which is 2.5 inches in diameter and 0.25 inch indepth) saturated with water. The fluid to be tested is poured along theinside edge of the filtration cell. The filtration test is thenconducted for 30 minutes at the desired temperature of 150° F. under apressure differential of 250 pounds per square inch supplied bynitrogen. The spurt loss is measured as the amount of fluid expelledfrom the filtration cell until the flow of fluid is reduced to drops.The fluid loss is measured as the total amount of fluid collected in 30minutes.

The Fann viscosity data is obtained utilizing a Fann 35 viscometer inaccordance with the procedures set forth in API Recommended PracticeRP-13B-1.

The typical particle size distribution of these particulate, sizedmagnesia products utilized in the examples to follow is set forth inTable B. The calcined magnesia products are available from TBC-Brinadd,Houston, Tex. The Activity Index of these products is a follows: MAG5-840 seconds; MAG 10-1410 seconds; MAG 20-1740 seconds. TABLE B TypicalVolume % of Particles Under the Indicated Size Particle Size, micronsMAG 5 MAG 10 MAG 20 3.09  26.81  17.49  10.23 5.03  43.36  27.71  17.535.86  50* — — 9.86  76.02  46.72  30.79 10.82 —  50* — 15.12  92.97 63.12  41.67 19.75 — —  50* 20.52  98.88  76.42  51.3 26.2 100  86.15 60.22 35.56 100  95.0  72.66 44 100  98.5  81.3 57.97 100 100  90.72106.8 100 100 100*Medium particle size (D₅₀)

The particle size of the magnesia is determined with a MalvernInstruments' MASTERSIZER particle size analyzer. The preferred particlesize of the calcined magnesia has an average particle size (D₅₀) fromabout 5 microns to about 50 microns.

The Activity Index of the calcined magnesia decreases as the particlesize decreases. The Activity Index of the calcined magnesia beforegrinding and sizing for the magnesia samples A, B, and C was greaterthan 40 minutes. Calcined magnesia having a median particle size (D₅₀)of 30, 50 and 150 microns has an Activity Index of 1890, 2940, and 5610seconds, respectively. The preferred calcined magnesia has an ActivityIndex from about 800 seconds to about 3000 seconds.

In order to more completely describe the invention, the followingnon-limiting examples are given. In these examples and thisspecification, the following abbreviations may be used: API=AmericanPetroleum Institute; LSRV=Brookfield low shear rate viscosity at 0.03revolutions per minute, 0.0636 sec⁻¹, in centipoise; sec=second(s);ppg=pounds per gallon; ppb=pounds per 42 gallon barrel; ° F.=degreesFahrenheit; ml=milliliters; min=minutes; cp=centipoise; rpm=revolutionsper minute; in=inches; sq.ft.=square feet; GS=gel strength.

EXAMPLE

Well drilling and servicing fluids were prepared containing 290.5 ml ofwater, 104 g (ppb) of NaCl, 7.0 g (ppb) BROMA FLA™ starch derivative, 10g (ppb) MAG 10, 30 g (ppb) MAG 20, and the concentrations of xanthan gumand citric acid set forth in Table 1. The properties after static-agingthe fluids for 16 hours at 150° F. were determined. The data obtainedare set forth in Table 1. The data indicate the excellent stability ofthe fluids and the increase of the LSRV by the addition of the citricacid.

The starch derivative BROMA FLA™ available from TBC-Brinadd, Houston,Tex. TABLE 1 Fluid 1-1 1-2 1-3 1-4 1-5 1-6 Xanthan Gum, g (ppb) 1.251.25 1.25 1.25 1.25 1.0 Citric Acid, g (ppb) 0 0.25 0.50 0.75 1.0 1.0Properties After Static Aging at 150° F. for 16 Hours, Test Temperature120° F. PV, cp 17 16 17 17 21 19 YP, lb/100 sq. ft. 28 38 37 42 49 4110-Sec GS, lb/100 sq. ft. 9 15 15 16 18 14 10-Min, GS, lb/100 sq. ft. 1118 18 19 23 17 LSRV, cp 16,700 44,100 38,400 43,100 62,200 31,000 pH11.1 10.9 10.7 10.7 10.5 10.3 Fluid Loss Spurt Loss, ml 3.0 2.0 1.5 2.02.0 2.0 30 min., ml 8.0 5.5 5.0 5.5 5.0 6.0

1. A well drilling and servicing fluid comprising an aqueous liquid, awater soluble polymer viscosifier, a particulate magnesia bridging agentwherein the magnesia has an Activity Index greater than about 800seconds, and citric acid.
 2. The fluid of claim 1 wherein the polymer isa biopolymer produced by fermentation of a carbohydrate source by theaction of bacteria or fungi which is an excellular polysaccharide havinga molecular weight in excess of about 500,000.
 3. The fluid of claim 2wherein the polymer is xanthan gum.
 4. The fluid of claim 1 furthercomprising a polymeric fluid loss control additive selected from thegroup consisting of pregelatinized starch, starch derivatives, cellulosederivatives, and mixtures thereof.
 5. The fluid of claim 4 wherein thepolymeric fluid loss control additive is a starch derivative selectedform the group consisting of hydroxyethyl starch, hydroxypropyl starch,hydroxyalkyl carboxymethyl starch, carboxymethyl starch, tertiaryaminoalkyl ether derivatives of starch, and the slightly crosslinkedderivatives of such derivatized starches, and mixtures thereof.
 6. Thefluid of claim 4 wherein the polymeric fluid loss control additive is ahydroxypropyl ether derivative of starch which has been slightlycrosslinked with epichlorohydrin.
 7. The fluid of claim 4 wherein thepolymeric fluid loss control additive is selected from the groupconsisting of a crosslinked ether derivative of (1) a partiallyhydrolyzed starch, (2) a partially depolymerized, crosslinked etherderivative of starch, and (3) mixtures thereof.
 8. The fluid of claim 6wherein the water soluble polymer is xanthan gum.
 9. The fluid of claim7 wherein the water soluble polymer is xanthan gum.
 10. The process ofdrilling a well wherein the fluid of claim 1 is circulated within aborehole being drilled as drilling proceeds.
 11. The process of drillinga well wherein the fluid of claim 2 is circulated within a boreholebeing drilled as drilling proceeds.
 12. The process of drilling a wellwherein the fluid of claim 3 is circulated within a borehole beingdrilled as drilling proceeds.
 13. The process of drilling a well whereinthe fluid of claim 4 is circulated within a borehole being drilled asdrilling proceeds.
 14. The process of drilling a well wherein the fluidof claim 5 is circulated within a borehole being drilled as drillingproceeds.
 15. The process of drilling a well wherein the fluid of claim6 is circulated within a borehole being drilled as drilling proceeds.16. The process of drilling a well wherein the fluid of claim 7 iscirculated within a borehole being drilled as drilling proceeds.
 17. Theprocess of drilling a well wherein the fluid of claim 8 is circulatedwithin a borehole being drilled as drilling proceeds.
 18. The process ofdrilling a well wherein the fluid of claim 9 is circulated within aborehole being drilled as drilling proceeds.
 19. The process ofcompleting or working over a well wherein a subterranean formation iscontacted with the fluid of claim
 1. 20. The process of completing orworking over a well wherein a subterranean formation is contacted withthe fluid of claim
 2. 21. The process of completing or working over awell wherein a subterranean formation is contacted with the fluid ofclaim
 3. 22. The process of completing or working over a well wherein asubterranean formation is contacted with the fluid of claim
 4. 23. Theprocess of completing or working over a well wherein a subterraneanformation is contacted with the fluid of claim
 5. 24. The process ofcompleting or working over a well wherein a subterranean formation iscontacted with the fluid of claim
 6. 25. The process of completing orworking over a well wherein a subterranean formation is contacted withthe fluid of claim
 7. 26. The process of completing or working over awell wherein a subterranean formation is contacted with the fluid ofclaim
 8. 27. The process of completing or working over a well wherein asubterranean formation is contacted with the fluid of claim
 9. 28. Amethod of increasing the low shear rate viscosity of a well drilling andservicing fluid which comprises adding citric acid to the fluid, whereinthe fluid comprises an aqueous liquid, a water soluble polymerviscosifier, a particulate magnesia bridging agent wherein the magnesiahas an Activity Index greater than about 800 seconds.
 29. The method ofclaim 28 wherein the polymer is a biopolymer produced by fermentation ofa carbohydrate source by the action of bacteria or fungi which is anexcellular polysaccharide having a molecular weight in excess of about500,000.
 30. The method of claim 29 wherein the polymer is xanthan gum.31. The method of claim 28 further comprising a polymeric fluid losscontrol additive selected from the group consisting of pregelatinizedstarch, starch derivatives, cellulose derivatives, and mixtures thereof.32. The method of claim 31 wherein the polymeric fluid loss controladditive is a starch derivative selected form the group consisting ofhydroxyethyl starch, hydroxypropyl starch, hydroxyalkyl carboxymethylstarch, carboxymethyl starch, tertiary aminoalkyl ether derivatives ofstarch, and the slightly crosslinked derivatives of such derivatizedstarches, and mixtures thereof.
 33. The method of claim 31 wherein thepolymeric fluid loss control additive is a hydroxypropyl etherderivative of starch which has been slightly crosslinked withepichlorohydrin.
 34. The method of claim 31 wherein the polymeric fluidloss control additive is selected from the group consisting of acrosslinked ether derivative of (1) a partially hydrolyzed starch, (2) apartially depolymerized, crosslinked ether derivative of starch, and (3)mixtures thereof.
 35. The method of claim 33 wherein the water solublepolymer is xanthan gum.
 36. The method of claim 34 wherein the watersoluble polymer is xanthan gum.